1. Field of the Invention
The present invention relates to golf balls. Specifically, the present invention relates to improvement of dimples of golf balls.
2. Description of the Related Art
Golf balls have a large number of dimples on the surface thereof. The dimples disturb the air flow around the golf ball during flight to cause turbulent flow separation. By causing the turbulent flow separation, separation points of the air from the golf ball shift backwards leading to a reduction of drag. The turbulent flow separation promotes the displacement between the separation point on the upper side and the separation point on the lower side of the golf ball, which results from the backspin, thereby enhancing the lift force which acts upon the golf ball. The reduction of drag and the enhancement of lift force are referred to as a “dimple effect”. Excellent dimples efficiently disturb the air flow. The excellent dimples produce a long flight distance.
There have been various proposals for the shapes of dimples. U.S. Pat. No. 7,250,012 discloses a golf ball which has dimples each having an annular tubular portion.
U.S. Pat. No. 6,558,274 (JP2001-54592) discloses a golf ball which has first dimples and second dimples. The second dimples are recessed from the first dimples.
U.S. Pat. No. 6,162,136 (JP2002-531232) discloses a golf ball which has dimples each having a central depression, a land ring and an annular depression.
US 2003/190968 (JP-2003-290390) discloses a golf ball which has dimples each having a projecting bottom. The curvature radius of the bottom is large.
US 2008/004137 (JP2008-12300) discloses a golf ball which has dimples each having a projection. The projection is surrounded by a ring-shaped recess.
The greatest interest to golf players concerning golf balls is flight distance. In light of flight performance, there is room for improvement in the shapes of dimples. An object of the present invention is to provide a golf ball having excellent flight performance.